This invention relates generally to the preparation of resist masks and more particularly to an electron beam lithography process.
Scanning, focussed electron beam systems such as are described, for example, in U.S. Pat. Nos. 3,644,700; 3,866,013 and 3,949,228 step an electron beam from one predetermined position to another to form an exposed pattern in a layer of radiation sensitive resist. The beam is preferably square-shaped and has a spot size equal to the minimum line width of the pattern which is to be formed. The beam is stepped across the surface of the resist in a raster fashion with the beam being blanked out where no exposure is desired. U.S. Pat. Nos. 3,866,013 and 3,949,228 describe a system which provides the capability of varying the time that the beam is turned on while it is at a given location. The stated purpose is to avoid over-exposure when the resist is over a material such as gold, which has a higher electron reflectivity than, for example, silicon or when a pattern line having a width equal to two spots of the beam is being formed. In the latter case, the developed shape will have a width greater than the width of two spots unless the exposure is reduced.
These exposure systems provide a stepping grid which is a fraction, for example one fourth, of the beam width. Shapes which are not even multiples of the beam spot size are formed by stepping the beam, with overlapping exposures, such that the total shape area desired is exposed.
Because the writing time of the pattern consumes a major portion of the resist mask formation process, it is desirable to have as large a beam or spot size as possible particularly when a large portion of the pattern is to be exposed. One constraint has been the need to have a beam size which is the width of the narrowest shape to be exposed. The choice of shapes is also limited to the stepping grid. A process has now been found which produces shape sizes which vary from the stepping grid and which can be smaller than the width of the beam.